Quantum Dots and Programmable Matter

Lead to Gold with the Click of a Mouse

Editor's note: You could say Wil McCarthy is a rocket scientist, so a discussion on quantum dots and programmable matter might seem like, well (you can guess where this is going) rocket science. But Wil makes the topic eminently accessible — and fascinating — in his article today. If you'd like to learn more about this "programmable matter," Wil plans to explore the social and technological implications of the quantum dot (a device capable of trapping electrons in a space so small that they form "artificial atoms" that can be controlled in real time) in his upcoming session at O'Reilly's Emerging Technology Conference.

For any inorganic material, and many organic ones, physics recognizes a triple point — a pressure/temperature sweet spot where solid, liquid, and gaseous phases can exist side by side. Strange occurrences are commonplace here, and peculiar structures abound. There's probably an alien planet somewhere out in the universe — as warm as the room you're sitting in now but as depressurized as the air 25 miles above your head — where arches of ice tower above boiling seas in an atmosphere of blue-gray fog.

The most complex organic material is called "humanity," though, and it has a triple point of its own. Or anyway I do, personally. As an aerospace engineer, I've worked with rockets, satellites, space probes, robots, computer vision systems, navigation systems, airships, and high-altitude balloons. I'm also a science fiction writer, constantly on the prowl for ideas that sound outrageous and dramatic, but that are actually physically possible. And I dabble on the side as a journalist and columnist at places like Wired and SciFi.com. Often, these three areas of my life are separated by firewalls as absolute as the distinction between crystal and vapor. But sometimes the boundaries blur and even, on rare occasions, vanish completely.

This happened in 1997, when I was reading a book on quantum dots and hit upon the idea of attaching them to electrically active fibers, and then weaving the fibers together. The result, which my business partner Gary dubbed "quantum wellstone," would be a bulk material whose apparent physical properties — its thermal, optical, electrical, and magnetic responses — were configurable in real time through the careful application of electrical voltages. "Programmable matter," if you will; click on a mouse and something that looks and feels and behaves like lead (except for its mass and ductility) can become, instead, something that looks and feels and behaves like gold.

Here was an idea that sounded crazy — sounded like magic, or alchemy — but had a firm basis in known science and emerging technology. It was the perfect fodder for science fiction. The only problem was that nobody believed it. My readers, long accustomed to accurate science, howled in outrage, and I was compelled to respond with a series of articles on the underlying science (itself a kind of triple point, where electronics and quantum mechanics and materials science meet), including commentary from the scientists themselves. The longest of these was "Ultimate Alchemy," a 7,000-word feature in Wired magazine (which is still available, minus the pictures, at www.wired.com/wired/archive/9.10/atoms.html).

But as the publication date for that article approached, Gary became increasingly uneasy. "This is too detailed," he said. "You're disclosing a patentable invention. Let's file the PTO paperwork first." And so we did, in August of 2001, and two years later, our jointly held aerospace research corporation, Galileo Shipyards, had spun off a subsidiary called The Programmable Matter Corporation, exclusively for the study of wellstone and related technologies, and we've been approached by half a dozen venture capitalists (whom we've turned away) and three government agencies (whom we haven't). Meanwhile, we're deep in talks with the purveyors of two wildly different fabrication technologies, both of which seem capable of producing an early, crude form of wellstone for use in the development of new materials. So, I've explored programmable matter in fiction, fact, and cutting-edge engineering. I'm at the triple point.

Historically, the properties of matter are determined at the time of manufacture, through careful mixing and processing. But now we find ourselves at the dawn of a new age, where substances exist whose optical, electrical, magnetic, and even mechanical properties can be adjusted at the flip of a bit. In a 75-minute lecture and 45-minute Q&A, engineer/journalist/novelist Wil McCarthy explores the social and technological implications of this "programmable matter."

If quantum dots and programmable matter are new to you, you're probably skeptical at this point, as well you should be. With a few notable exceptions, science fiction writers aren't known for inventing crazy gadgets that actually work. And wellstone doesn't — yet. But it should, and to help you understand why, I'll take a step back and summarize, in layman's terms, the physics behind dynamically configurable materials.

First of all, here's a two-word lesson in materials science: electron clouds. Every atom has one, and every molecule or crystal has an intermingling of the clouds of its individual atoms. These clouds sometimes have sticky arms that readily attach to other clouds. Sometimes there are empty spaces in the cloud through which electrons can travel with little resistance. And like the water clouds in Earth's atmosphere, electron clouds can exist in multiple layers above the atomic nucleus. When an electron jumps to a higher layer, it absorbs energy (in the form of heat, light, mechanical vibration, and so on), and when it falls to a lower level, it emits energy (usually in the form of light). Ignoring the subject of magnetism, which is a bit more complex, this oversimplified model explains (well, hand-waves) most of the important electrical, optical, chemical, and thermal properties of materials.

The electron cloud of an ordinary atom is held in place by the positive electrical charge of the protons in the nucleus. Electrons are negatively charged, and opposites attract. But the cool thing is, you can create an electron cloud — an artificial atom — that doesn't have a nucleus of its own. This miracle is accomplished with quantum dots, which are electronic components that trap electrons in a space so small that their quantum (wave-like) behavior dominates over their everyday classical (particle-like) nature. The electrons trapped in a quantum dot are really roaming through the cloud structure of the semiconductor crystal from which the dot is made, but they don't necessarily know that. And because they obey a quantum-mechanical rule called the Pauli Exclusion Principle, they arrange themselves in the same sorts of patterns as the electrons in real atoms. They behave, in many ways, like oversized, under-energized atoms.

What's more, by controlling tiny electrical fields around these dots, we can pump electrons in and out of them, by ones and twos, or by hundreds and thousands. We can even control the size and shape of the cloud they form. Thus, they can be herded into structures that mimic the properties of natural atoms, or that have completely novel properties that don't occur in nature.

This is being done right now, in corporate and university laboratories all over the world. Unfortunately, quantum dots' applications in computing and optics are currently overshadowing their more dramatic (indeed, transforming) implications for materials science. But progress is being made, and the writing is on the wall: by arranging these components in two-dimensional arrays, we can dramatically alter the surface properties of a material. And by rolling these surfaces into fibers, we can create a material that is mostly surface, and whose bulk, interior properties can be controlled on demand, to suit the needs of the moment.

Is there a single field of human endeavor that wouldn't be affected by a technology like that? Technically speaking, "programmable matter" is any bulk substance whose physical properties can be adjusted in real time through the application of light, voltage, electric or magnetic fields, and so forth. Such materials exist right now, but allow only limited adjustments of one or two traits (for example, the "photodarkening" or "photochromic" materials found in light-sensitive sunglasses). In ordered arrays, these materials can produce programmable surfaces like the LCD screen of your laptop, whose appearance can be altered dramatically and instantly. But quantum dots offer a much greater promise: materials whose very substance can be changed as easily as a picture on a screen.